1. Field of the Invention
This invention relates to carbon fibers, and more particularly, to an improved process for stabilizing polyacrylonitrile (PAN) based fibers prior to carbonization and high-temperature heat treatment.
2. Description of the Prior Art
There has been an increasing demand for construction materials of high strength-to-weight ratio and high Young's modulus for use in civilian and military applications, particularly for materials that have good thermal stability. Accordingly, there has been a demand for improved reinforcing fibers to be embodied in structural composites which are exposed to conditions of high temperature and high mechanical stress.
Carbon fibers are produced in five manufacturing steps: polymerization of monomers, such as acrylonitrile; orientation of the linear polymer molecules by spinning them into fibers; stabilization by heating the fibers to 150.degree.-350.degree. C.; carbonization at 1000.degree.-1500.degree. C.; and high-temperature heat treatment above 2000.degree. C. to produce high-modules carbon fibers.
During the stabilization process, PAN filaments are heated to 150.degree.-350.degree. C. in air for several hours under tension. Volatile components, including ammonia, HCN, carbon monoxide, carbon dioxide and light hydrocarbons are evolved, and oxygen from the air reacts with the polymer. The linear polyacrylonitrile polymer molecules cross-link, forming stable filaments. During the subsequent carbonization step, hydrogen, oxygen, and nitrogen-containing compounds are progressively pyrolized, leaving eventually fibers of essentially pure carbon. The microstructure of the stabilized fiber precursor has bearing on the structure and mechanical properties of the final carbon fiber product.
Numerous methods of conducting the stabilization of polymer fibers aimed at improving the properties of the resulting carbon fiber are known in the prior art. Turner (U.S. Pat. No. 3,862,334) conducts the stabilization between 200.degree. and 300.degree. C. in an inert atmosphere and then continues in an oxygen-containing atmosphere between 150.degree. and 250.degree. C. Shindo (U.S. Pat. No. 3,529,034) stabilizes cellulosic, polyvinyl alcohol, or acrylic fibers at elevated temperatures in presence of hydrogen chloride vapor. Riggs (U.S. Pat. No. 3,961,888) stabilizes polyacrylonitrile fibers first in an inert atmosphere and later in an oxygen-containing atmosphere. Yoshida et al. (U.S. Pat. No. 4,256,607) produces activated carbon fibers with properties similar to activated carbon black by first stabilizing acrylic fibers in air at 200.degree.-300.degree. C. and then activating the fibers at 700.degree.-1000.degree. C. in presence of ammonia, carbon dioxide or water vapor. Warner (U.S. Pat. No. 4,295,844) stabilizes acrylic fibers in air at 200.degree.-360.degree. C. after contacting them with aniline. Cagliostro (U.S. Pat. No. 4,385,043) stabilizes polyacrylonitrile fibers at elevated temperature in an oxidizing atmosphere and then in an inert atmosphere into which acetylene is added.